The present invention relates to coating compounds for forming self-coating or self-layering (self-stratifying) lacquer systems, for use in particular in the automobile industry, as well as a method for manufacturing these coating compounds.
Surfaces or coatings for parts or components of transportation means, for example motor vehicles, airplanes, rail-mounted vehicles, etc., must satisfy extreme demands with respect to corrosion protection, scratch resistance, optics, coloring, chemical resistance, and resistance to other possible environmental effects.
This places great demands on the materials utilized, which must at the same time be manufactured cost-effectively. This is particularly true in the automobile industry, where parts manufacturing processes are highly competitive and where the coating materials and their methods of application must satisfy increasing environmental compatibility demands.
Coating materials that are able to cope with strict environmental standards, especially in regard to avoiding the need for organic solvents that are manufacturable and usable cost-effectively have therefore long been sought. This also includes self-stratifying coating compositions, which are based both on incompatible resin powder mixtures and on incompatible lacquer resins, dissolved in organic solvents.
Such compositions were described for example in a publication that appeared in the periodical Industrie-Lack 44 (1976), No. 8, page 305. Additional examples can be found in the XV. FATIPEC Congress Book, II (1980), 387 or in the periodical, Progress in Organic Coatings (1996), 30 Jul., a special edition devoted to self-stratifying coatings. So as to avoid mere repetition of each of these three disclosures, they are incorporated in their entirety into the present application, in particular with regard to the materials and manufacturing processes for self-stratifying compositions.
The basics of self-layering lacquer systems based on incompatible powder mixtures and on incompatible lacquer resins dissolved in organic solvents are likewise described in the publications mentioned above, as well as in the research report of the Paint Research Association “Self-Stratifying Coatings” March 1995.
British Patent GB 2 046 765 describes self-coating compositions based on powdered paint resins and film-forming resins in organic solvents, which produce multilayered film coatings.
A powder coating process is described in British Patent Application GB 1 570 540 which can be produced using multilayered coatings. Further publications, which describe the manufacture of multilayered coatings using powdered coating compositions are German Patents DE 28 21 012 C2, DE 30 08 825 C2, and DE 30 30 116 C2, and U.S. Pat. No. 4,305,994. All these publications deal with processes for manufacturing coating compounds that may be utilized for various purposes. German Patent 31 17 302 C2 describes a process for manufacture of a multilayered olefinic resin film on a metal substrate.
However, a disadvantage of all these processes is that as a result of their high viscosity the two powder lacquers separate only slowly from the melt, and extremely high temperatures are needed for the two incompatible molten powder coatings to be able to separate. The cycle times for separation achievable with this lacquer system make an application as self-layering (self-stratifying) or self-coating lacquer systems, on the basis of two incompatible powder lacquers, impossible for mass production in the automobile industry.
A further disadvantage is the high temperature that is necessary for effective separation; therefore, with the different substrate materials utilized in building automobiles, ranging from steel to low melting thermoplastics and mixtures thereof, such a composition cannot be used.
British Patent GB 2 192 399 describes coating compositions, which are mixtures of copolymers in organic solvents. The copolymers used are fluorine- or silicone-containing copolymers having active hydrogen, and acrylate copolymers having active hydrogen in various mix ratios. Although this publication describes use of these compositions for automobile lacquering, their use is very doubtful based on pollution control considerations, since extremely poisonous organic solvents such as xylene, methylisobutyl ketone, or Solvesso 100 or 150 (a C10-15 aromatic mixture) are utilized, and formulations which are obtained are of critical environmental concern. The same is true for the coating compositions described in European Patent Application EP 1 038 935 A1 (WO 9920702 A1).
The use of different self-coating resin systems based on organic solvents which were described in the March 1995 research report by the Paint Research Association “Self-Stratifying Coatings” and which are described in the results of the same named BRITE/EURAM research project of the same name from 1989–1993, are likewise questionable on the grounds mentioned above.
German Patent DE 37 02 503 C23 describes a method for formation of a multilayered film on a substrate, wherein a cationically-depositable, epoxy-type electrocoating material, following neutralization with a suitable acid forms an aqueous bath suitable for cationic deposition, is mixed with a non-ionogenic film-forming acrylic, polyester resin, polyester-modified resin, or silicone-modified resin and is introduced into an electrodeposition bath. An electrically conductive object is immersed in the bath and is used as the cathode. A film is deposited under the conditions described, and is finally hardened at 150–230° C. to form a multilayered film.
This coating system should function as a corrosion protection base or primer for subsequent lacquer build-up (protective paint; base coat and clear lacquer). Such a coating material is not, however, suitable as a post-primer for automobiles, because due to the various colors an immersion-based process is not practicable, since for each color a separate coating bath having the desired dipping lacquer must be available. Aside from that, the epoxide systems, which are used do not meet the stability requirements necessary in protective painting in automobile construction.
Two known coating lacquers in vehicle lacquering are described below as examples of a sequence of coatings having different functions, it being impossible to list all possibilities regarding variants and alternatives in the lacquering operation and the construction of multilayered systems for different substrate materials such as steel, plastic, and light metal. One possibility of lacquering is to phosphatize a substrate layer, then to treat this pre-treated material with a cathodic immersion lacquer, coat it with a filling material, apply a base lacquer coat and subsequently to provide a clear lacquer.
Another possibility for forming a lacquer coating is to coat a phosphatized substrate coat likewise with a cathodic dipping lacquer and then to apply a layer of a hydrated filler material and subsequently treat with a pigmented protective paint. These incompatible lacquer systems based on powder resin mixtures or on organic solvents are known as self-stratifying coatings.
All processes known to date, however, have in common that they either are not applicable to the automobile industry or cannot be used due to environmental concerns. There is therefore a great demand for a lacquer system that can be utilized universally based on the use of water as solvent. However, none of the attempts that have been made to date to develop a coating composition based on water have been successful.
Self-coating or self-layering (self-stratifying) coating compositions for manufacturing multilayer coatings have a series of characteristics of considerable economic and environmental interest.